1. Field of the Invention
Our invention relates to a recording and reproducing method, and in particular to such a method employing a pulse-code modulation (PCM) scheme. Still more particularly, our invention deals with a PCM recording and reproducing method well calculated to permit distortionless reconstruction of the original signal in spite of possible dropout errors. The method of our invention is disclosed herein as adapted for recording and reproducing audio signals on and from recording media such as magnetic tape or laser discs, but with no unnecessary limitations thereto being intended.
2. Description of the Prior Art
The processing of audio and video signals by the PCM technique has been known and practiced for some time now. With PCM, the signals can be recorded and reproduced with an extra-ordinarily high degree of faithfulness if no errors are introduced into the signals themselves, because the signal quality is unaffected by the qualities of the transmission media through which they journey. In practice, however, the signals are easy to suffer errors.
If, for example, foreign particles are attached to recording media such as magnetic tape, or if the recording media have some surface imperfections, then the signals will not be properly recorded on or reproduced from such defective regions of the recording media. These are causes for the problem known as dropouts. If a dropout error occurs to even one bit of a PCM word, the word may become unable to correctly represent the corresponding sample, or instantaneous value, of the original analog signal, possibly introducing noise into the information reproduced.
A variety of methods have been suggested and used for the correction or elimination of dropouts. The following four methods are among those best known: (1) to doubly or multiply record the same PCM signal and to reproduce either of the recordings having no dropout error; (2) to record an analog signal in both PCM and analog format and, upon detection of a dropout error in the PCM signal, to use the corresponding portion of the analog signal; (3) to interpolate an approximate value computed from the values preceding and succeeding the lost value; and (4) to compensate for the lost value by holding the preceding value.
The problem of dropouts can be overcome, to a considerable degree, by the use of the above enumerated methods, either singly or in combination of two or more. Each of these conventional methods has its own drawbacks, however.
Method (1), when employed singly for dropout elimination, necessitates the recording of the same PCM signal a considerable number of times. Method (2) requires the provision of additional equipment and an additional track for recording an incoming analog signal in analog format, and further precise synchronization must be realized between the analog and the PCM signals. According to method (3), complex circuitry is required for the computation of approximate values to be interpolated. This third method, moreover, is incapable of accurately compensating for a plurality of values lost consecutively. Method (4) permits easier compensation but is also incapable of accurately compensating for a plurality of values lost consecutively.